The functional equivalence has always been the integral part of virtually every logic synthesis tool. The formal equivalence checking represents a key process that helps logic synthesis tool guarantee that two representations of a circuit design exhibit exactly the same behavior. Among others,equivalence checking is routinely applied to prove that a synthesized digital circuit is logically equivalent to the RTL source code. Although formal equivalence checking has matured greatly during the last two decades and designs with millions of gates can be handled and verified in reasonable time, a new challenge has emerged with the recent advent of approaches addressing the problem of synthesis of approximate circuits.